A huge advantage of the rack for rappelling is that it's a variable friction device. You can compress bars together, or add bars on open frame racks, to increase friction. Let the device do the work, not the death grip of your hand or body friction. If you're wrapping the rope around your butt, you're doing it wrong - or at least not optimizing the device you're using. However if you're using something like a figure-8 descender, different story.

OK ... I feel compelled to offer a situation and see if anyone has additional device.

I think that for better or worse ... this can happen to anyone ...

Caver with a standard rack is caving with someone whose rope is 1/2 dirty. I'm picturing the clean 1/2 of the rope is rigged on bottom 1/2 of pitch and dirty part is on top half of pitch. Now ... person is using ON-ROPE approach and feeding rack from close distance.

Friction changes dramatically on rope 1/2 way down and caver pinches finger into rack because feeding from 12 inches or 8 inches away from rack. Caver lets go. Not sure this ever happened to me. But I am sure I've been on rope many many times when friction changes dramatically. Also fairly sure I have been glad to be applying BUTT-friction at the time. As an eight user ... (shorter pitches) ... I might be recalling an 8 experience. Can't say for use.

Anyways ... I didn't know that proper use of a rack was also license to feed rack from closer and pinch fingers. I'm open minded. Maybe it should be exactly as you describe. I am guilty sometimes of using prob too few bars (4).

Not at all want to get flamed or flame anyone. And I know that we all assume plenty of risk in any approach we take in caving and climbing.

I've never "fed" the rack at all except at the top of pitches. I let the device do its job. As I understand it, the "control" hand cradles the rack and slides/removes/adds bars. I let the rope run through my right hand, probably a foot or so below the rack, but I do not grip it. I use an 8 or a Munter more often than a rack, and never butt-wrap these either. I weld my wrist to my hip and let my grip do the work. Of course I am very insubstantial, bodily, and can get away with a lot...

I don't know what's "right," but I reckon safety has a lot more to do with being aware of and prepared for potential issues than it does any certain technique. So your scenario, alternately dirty/clean or dry/wet rope, is quite useful.

I was never taught that rope around the waist was mandatory, but that control of rope below rack was important taking into consideration rack configure plus skills of said rack user. I have let go of the rope with my 'control' hand (hand below rack). Rack configuration and what some would call 'tuning' can mean a great deal in control ability or lack thereof.